1. Field of the Invention
The present invention relates to a pulse-signal extracting method and apparatus for removing a noise from an input signal, where a low-frequency component is introduced as the noise into a pulse waveform, so as to derive the pulse waveform.
2. Description of the Prior Art
FIG. 7 shows a prior art arrangement of a semiconductor integrated circuit having a function to derive a pulse signal from a signal where a low-frequency component is superimposed as a noise on a pulse waveform. In FIG. 7, to a comparator 1 there are inputted an input signal PulseIn (which will be referred hereinafter to as a pulse input signal) A where a low-frequency component is superimposed as a noise on a signal component having a pulse waveform and further inputted a comparison voltage VREF (which will be referred hereinafter to as a reference voltage) B. The reference voltage B is set to be an optimal constant DC voltage Eo. Further, the comparator 1 generates a pulse-waveform output PulseOut (which will be referred hereinafter to as a pulse output) C. The comparator 1 compares the level of the reference voltage Eo with the level of the pulse input signal A so as to generate a "H" level when the former is higher than the latter and generate a "L" level when the former is lower than the latter.
Secondly a description will be made hereinbelow in terms of the operation. FIG. 9 shows a pulse output C obtained when a pulse input signal A and a reference voltage Eo illustrated in FIG. 8 are inputted to the comparator 1 and further shows an expected pulse output D. As illustrated in FIG. 8, the pulse input signal A is a pulsating-current signal where a low-frequency component is superimposed as a noise, and the reference voltage Eo is inputted as a constant voltage. Thus, the comparator 1 outputs the pulse output C as illustrated at the upper side of FIG. 9. Here, the pulse output C is required to include pulses mP and mQ in an interval from a to b as being similar to the expected pulse output D illustrated at the lower side of FIG. 9. However, since the pulse input signal A becomes a pulsating current signal due to the superimposition of the low-frequency component to generate an interval where the level of the pulse input signal A becomes lower than the level of the reference voltage Eo, the pulses mP and mQ expected are not generated from the comparator 1.
Since the amplitude S of the pulsation due to the noise is relatively large as compared with the height h of the pulse of the signal component, that is, since the the entire pulse input signal greatly pulsates due to the noises which is the low-frequency component, there is a problem that the pulse waveform required is not outputted because of no detection of the pulses or that difficulty is encountered to adequately set the comparison voltage to allow the detection of the pulses.